1. Field of the Invention
The present invention relates to an ATM (Asynchronous Transfer Mode) layer device for realizing the function of an ATM layer in an ATM transmission network.
2. Description of Related Art
In an ATM transmission network adopting the B-ISDN (Broadband Integrated Services Digital Network) standard and others, a protocol for transferring information comprises a plurality of layers. Respective layers have inherent functions, and one layer is realized by utilizing the function of a layer inferior to the first named layer by one layer.
Of the plurality of layers, a lower layer, which is the most remote layer from a user""s side, includes a physical layer and an ATM layer.
The physical layer is a layer for providing a resource for transferring a cell, which is a unit of information transmission. Specifically, the physical layer generates a signal format suitable for a transmission medium, receives a signal, encodes, and/or generates and eliminates a transmission frame. Furthermore, the physical layer checks whether or not the information included in the cell is effective, and transfers the cell carrying the effective information (effective cell) to the ATM layer.
The ATM layer is a layer superior to the physical layer by one layer, and executes a multiplexing and a separation of cells.
An ATM layer device, which is a device realizing the function of the ATM layer, and a physical layer device, which is a device realizing the function of the physical layer, are connected through, for example, a UTOPIA interface (an interface standard, Universal Test and Operation PHY Interface for ATM).
FIG. 7 is a block diagram illustrating the connection between the ATM layer device and the physical layer devices.
As shown in FIG. 7, xe2x80x9cnxe2x80x9d physical layer devices 1021 to 102n (where xe2x80x9cnxe2x80x9d is a positive number) are connected to an ATM layer device 101 through a UTOPIA interface. The ATM layer device 101 selects one from the xe2x80x9cnxe2x80x9d physical layer devices 1021 to 102n, and transfers a cell between the ATM layer device and the selected physical layer device. Incidentally, the prior art ATM layer device gives an equal opportunity of selection to all the physical layer devices 1021 to 102n, since a priority order is not provided among the physical layer devices 1021 to 102n.
A system for equally selecting the physical layer devices 1021 to 102n includes, for example, a xe2x80x9cround robinxe2x80x9d scheduling. In this xe2x80x9cround robinxe2x80x9d scheduling system, as shown in FIG. 8, all the physical layer devices 1021 to 102n are selected in a predetermined order, as if a closed loop were depicted.
In the UTOPIA interface standard, on the other hand, since a transmission rate of the cell is not defined, a physical layer device having a high transmission rate and a physical layer device having a low transmission rate are connected to the ATM layer device in a mixed condition.
For example, consider a case in which a plurality of physical layer devices exists, each device having a low transmission rate and simultaneously receiving the cell, and in which one physical layer device having a high transmission rate exists and constantly receives the cell.
In this case, if the physical layer devices are selected in accordance with the round robin scheduling, after the ATM layer device receives the cell from the plurality of physical layer devices having the low transmission rate, the ATM layer device receives the cell from the physical layer device having the high transmission rate.
Accordingly, before the physical layer device having the high transmission rate transmits the cell to the ATM layer device, it receives a next cell, so that there is a possibility that a receiving buffer for temporarily storing the cells might overflow.
Even if the receiving buffer does not overflow, the actual transfer rate becomes smaller than the expected transfer rate because of the waiting time required for transferring the cell to the ATM layer device.
Accordingly, it is an object of the present invention to provide an ATM layer device which overcomes the above mentioned problem of the prior art.
Another object of the present invention is to provide an ATM layer device capable of preventing drops in the cell transfer rate and overflows of the receiving buffer provided in the physical layer device.
The above and other objects of the present invention are achieved in accordance with the present invention by an ATM layer device controlling method realizing the function of an ATM layer by selecting one from a plurality of physical layer devices realizing the function of a physical layer, and executing a transfer of a cell, which is a unit of transmission, between the ATM layer device and the selected physical layer device, the method including the steps of holding the priority information indicating the previously set priority level of each of the plurality of physical layer devices, and selecting the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information.
At this time, it is possible to output the address of the physical layer device which should execute the cell transfer, selected in accordance with the priority information, and to execute the cell transfer between the ATM layer device and the physical layer device designated by the address. Alternatively, it is also possible to output the address of the physical layer device of a candidate which should execute the cell transfer, in accordance with the priority information, and to respond to a cell transfer enable notify signal indicating that the cell transfer is possible, outputted from the physical layer device designated by the address, so as to output the address of the physical layer device that should execute the cell transfer.
Preferably, the priority is set such that the larger the transmission rate of the physical layer device, the higher the priority level of the physical layer device.
According to another aspect of the present invention, there is provided an ATM layer device configured to realize the function of an ATM layer by selecting one from a plurality of physical layer devices realizing the function of a physical layer, and executing a transfer of a cell, which is a unit of transmission, between the ATM layer device and the selected physical layer device, the ATM layer device including a priority information register holding the priority information indicating the previously set priority level of each of the plurality of physical layer devices, a selecting physical layer address output circuit for outputting the address of the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information, and a control circuit for executing the transfer of the cell between the ATM layer device and the physical layer device designated by the address.
In one embodiment, the ATM layer device further includes a polling physical layer address output circuit for outputting, in a predetermined order, the address of the physical layer device of a candidate that should execute the cell transfer, selected in accordance with the priority information, and wherein the selecting physical layer address output circuit responds to a cell transfer enable notify signal indicating that the cell transfer is possible, outputted from the physical layer device designated from the address, so as to output the address of the physical layer device that should execute the cell transfer.
Preferably, the selecting physical layer address output circuit can include a plurality of priority control registers for holding the information of the physical layer devices in units of different levels of priority, and the polling physical layer address output circuit can include a plurality of priority control registers for holding the information of the physical layer devices in units of different levels of priority.
Furthermore, it can be set such that the larger the transmission rate of the physical layer device, the higher the priority level of the physical layer device.
With the above mentioned arrangement, the ATM layer device in accordance with the present invention holds the priority information indicating the previously set priority level of each of a plurality of physical layer devices, and selects the physical layer device that should execute the cell transfer, in a predetermined order in accordance with the priority information. Therefore, the higher the priority level of the physical layer device, the larger the opportunity for selection of the physical layer device becomes, so that the data transfer amount per unit of time of a physical layer device having a high priority level can be increased.
The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention with reference to the accompanying drawings.